Studies are being conducted on the growth and maturation of autonomic nerve fibers in the hearts of fetal and neonatal rabbits. Specific areas of interest include the appearance of catecholamines in sympathetic adrenergic nerve fiber, the establishment of potentially functional interrelationships or contacts between autonomic nerve fibers and regular cardiac and specialized conduction muscle cells and peripheral interrelationships between adrenergic and cholinergic nerve fibers and effector muscle cells. This project is part of a continuing study of the ultrastructure and histochemistry of the autonomic nervous system during ontogenesis. Techniques being used include the histofluorescent method for catecholamines, electron microscopy, and histochemical methods for the localization of acetylcholinesterase at the fine structural level. New Zealand white rabbits ranging in age from eighteen days of gestation to six weeks postnatal are being used.